As an input medium, a touch panel has been the simplest, most convenient and natural human-machine interaction manner, and hence has been increasingly applied to various electronic products, such as mobile phones, laptops, and MP3/MP4 players.
As shown in FIG. 1 which is a schematic diagram showing the structure of a mutual capacitive touch panel in the prior art, the touch panel includes a plurality of driving lines (such as driving lines Y1 to Y4) and a plurality of sensing lines (such as sensing lines X1 to X4) intersecting orthogonally with the plurality of driving lines, and one of sub-pixels in the touch-screen is shown within the dotted box. The capacitance generated at the intersection part between the driving line and the sensing line cannot be changed by an external touching object, but outputs steady background noise or a Direct Current (DC) component to a preamplifier A. The spatial fringe electric field produced by the non-electrode intersection part between the driving line and the sensing line forms a mutual capacitor Cm, which would be directly affected by the external touching object.
The working principle of the typical mutual capacitive touch panel shown in FIG. 1 is briefly described as follows. Driving signals having a specific frequency are sequentially inputted via ends of the driving lines, and the preamplifier A connected to an end of each of the sensing lines receives and amplifies a signal having the same frequency which is sensed by the mutual capacitor Cm between the driving line and the sensing line. When a finger touches the surface of the touch panel, parasitic capacitors are formed between the finger and the driving electrodes as well as between the finger and the sensing electrodes, and a part of the signal having the same frequency would be leaked from the parasitic capacitors to the ground directly through the body or the grounded object, so that the signal to be received by the preamplifier A is attenuated. Based on the design for the electrodes of the touch panel, the driving signal frequency, and the distance between the finger and the electrode of the touch panel, the driving signal may also be coupled from the driving line to the sensing line through the finger as a medium, thus enlarging the signal to be received by the preamplifier A. In such two signal sensing modes, the specific position touched by the finger T can be found by sequentially detecting the signal change on each of the sensing lines.
However, the mutual capacitive touch panel shown in FIG. 1 cannot detect all touch signals. Reference is made to FIG. 2, which shows experimental curves of response characteristics versus the distance between the finger and the electrode with respect to the mutual capacitive touch panel shown in FIG. 1 under a certain driving signal frequency. As shown in FIG. 2, a curve F indicates the capacitance of the mutual capacitor Cm versus the distance between the finger and the electrode, and a curve Fl is the differential of the curve F, indicating the touch sensitivity. When the finger is close to the electrode, the capacitance of the mutual capacitor Cm decreases with the increase of the distance between the finger and the electrode. When the finger is at a certain distance to the electrode, the capacitance of the mutual capacitor Cm decreases to be close to the background noise value, and then, the capacitance of the mutual capacitor Cm is increased to a certain value with the increase of the distance between the finger and the electrode. Accordingly, the curve F is divided into both positive and negative parts, and it can be considered that the touch panel has positive and negative operating modes. However, when the capacitance of the mutual capacitor Cm is very close to the background noise value, the change of the capacitance of the mutual capacitor Cm is undetectable by the touch panel in both of the positive and negative operating modes, and such a region is called a blind zone for the detection.